The present invention relates to an electronic tripping device for low-voltage circuit-breakers and in particular for low-voltage multipolar automatic circuit-breakers for the protection of electrical networks.
Automatic circuit-breakers that perform a function of protection against ground faults use circuits for measurement or reconstruction of the value of the summation of network currents. Normally, an electrical network that is to be protected is made up of electrical conductors, corresponding generally to three phases and to the neutral of an installation. Contacts for closing and opening the circuit enable circulation or interruption of the current in the phase conductors. Normally, in most installations, the neutral conductor is distributed without being interrupted.
A plurality of current transformers T1, T2, T3, and TN associated to the various conductors of the network transform the primary currents of the circuit into secondary currents that are compatible with the electronic contact breakers. The secondary currents are then applied to the input of a circuit for rectification and detection of the phase, neutral, and ground-fault currents. This circuit generally supplies signals representing these currents to an electronic processing device. According to the conditions that arise, the processing device produces a tripping signal, which is applied to the input of a relay control, which in turn actuates a mechanism for opening the contacts of the automatic circuit-breaker.
In some tripping devices of a known type, for example the ones described in EP-A-284 198, measurement of the ground-fault current is made by means of a transformer, the primary winding of which is constituted by all the conductors of the electrical network. The secondary winding of the transformer hence supplies a current proportional to the summation of the currents of the network. A signal representing this secondary current Ih1 is supplied, for example, by the voltage Vh1 to the terminals of a resistor Rh1, which is connected in parallel to the secondary winding of the transformer. The sum of the secondary currents is obtained in a common conductor S1, connected to which is a first end of each of the secondary windings of the current transformers, the second ends of said secondary windings being connected directly to a rectifier circuit. The common conductor is connected, via measurement means, to the rectifier circuit. A current representing the ground-fault current, corresponding to the sum of the secondary currents of all the transformers, thus circulates in the conductor S1. The measurement means can be constituted by a resistor Rh2, the voltage Vh2 at the terminals of which representing the current summation.
In contact breakers that use the effective value RMS of the currents, a total independence of the currents is necessary. In such an embodiment, the two ends of the secondary winding of each current transformer (T1, T2, T3, TN) are connected to the rectifier circuit. The set of the secondary windings constitutes the primary winding of a transformer TSM, which supplies to the terminals of its secondary winding a voltage Vh4 representing the current summation in the network. The signals Vh1, Vh2, Vh3, or Vh4 are then applied to a processing device. In other automatic circuit-breakers, such as for instance the ones described in the patent application No. EP-A-0179017, digital processing circuits calculate a value representing the ground fault as a function of the amplitudes of the current values of the phases and of the neutral.
In other cases, such as for example in the U.S. Pat. No. 4,947,126, a measurement device is used, in which a datum is introduced on the polarity for the purpose of providing ground protection.
For reconstruction of the ground-fault signal, known ground-protection devices use electronic components which occupy considerable space on printed circuits. These components are frequently power transformers, diodes, or resistors. The cost and size of these supplementary power components are considerable for contact breakers installed in low-calibre automatic circuit-breakers.
Digital-processing contact breakers reduce the number of power components, but sampler/limiter circuits are necessary for simultaneous measurement of the values of the currents. Furthermore, the known algorithms for calculation of the value of the ground-fault current are not suited to all the configurations of phase and neutral currents, especially in the case where the currents do not present a pure sinusoidal form.